Thursday 31 May 2007 04.41 EDT
First published on Thursday 31 May 2007 04.41 EDT

Stress experienced by a woman during pregnancy may affect her unborn baby as early as 17 weeks after conception, with potentially harmful effects on brain and development, according to new research. The study is the first to show that unborn babies are exposed to their mother's stress hormones at such an early stage in pregnancy.

The findings, published in the journal Clinical Endocrinology, come after separate research on animals showed that high levels of stress in a mother during pregnancy could affect brain function and behaviour in her offspring, and other evidence suggesting that maternal stress in humans can affect the developing child, including lowering its IQ.

However, the way this happens and the implications for the unborn child, both before and after birth, are still not fully understood and further research is needed, the latest study's authors said.

They said they did not wish to "unduly worry pregnant women", but highlighted the need to lead a "healthy, balanced lifestyle" to avoid general stress.

The baby charity Tommy's called on family, friends and employers of pregnant women to provide support and reassurance to help them reduce stress.

The findings, the latest to focus on the impact of the environment in the womb on later development, come days after the government changed its advice to pregnant women and those trying to conceive, warning them to abstain from drinking alcohol. Previous guidelines had said they could drink up to two small glasses of wine a week.

The change in advice, which government health advisers said was made to avoid confusion, rather than in response to new medical evidence, prompted claims from some critics that pregnant women are increasingly becoming targets in an obsessively anti-risk culture.

Researchers in the latest study, led by Professor Vivette Glover at Imperial College London and the consultant obstetrician Pampa Sarkar, from Wexham Park hospital, Berkshire, measured levels of the stress hormone cortisol in 267 pregnant women. Cortisol, which is pumped into the blood when we become anxious, is good in the short term, as it helps the body to deal with a stressful situation, but long-term stress can cause tiredness, depression and make an individual more prone to illness.

Scientists sampled blood from the mother and amniotic fluid from around the foetus in the womb and found that, at a gestational age of 17 weeks or greater, higher cortisol levels in the mother's blood were reflected in higher levels in the amniotic fluid. Amniotic fluid is mainly produced by the foetus and is a good indicator of its exposure to a range of substances, including hormones.

Dr Sarkar said further research was needed into how high levels of stress in a mother affect the unborn baby. "We are all a product of our developmental history," she added. "One of the times when we are most susceptible to the influences of our surrounding environment is when we are developing as a foetus.

"Our research shows that the foetus is exposed to cortisol in the maternal blood, and we also demonstrated that at and above 17 weeks, the cortisol in amniotic fluid had a strong positive relationship with cortisol in maternal blood. We found that the strength of this correlation became stronger with increasing gestational age. We now need to carry out further work to unravel the mechanisms by which maternal stress affects the foetus, both during foetal life and through into childhood.

Claire Friars, a midwife at the charity Tommy's, said: "This is an important study as, for the first time, there's solid evidence to show that an unborn child may be exposed to maternal stress as early as 17 weeks in development.

"A crucial next step would be to uncover to what extent different levels of maternal stress can potentially affect an unborn child. For now - based on previous research - one thing is clear: high levels of stress in pregnancy can in some cases be detrimental to the health of the baby.

"To remain as stress-free as possible is certainly important during pregnancy. Of course, this is easier said than done, as pregnancy itself can incite all sorts of feelings - from feeling overwhelmed, happy and nervous. Pregnancy can signify major emotional changes in mums-to-be, from mood swings to feeling incredibly anxious, which may well elevate women's stress levels.

"It is vital that pregnant women are given adequate support and reassurance from their family, friends and employers, to ensure they have a happy and healthy pregnancy."

An earlier study, published in January and led by Prof Glover, measured the intelligence of more than 100 babies and toddlers whose mothers had suffered unusually high stress in pregnancy. It found their IQ was generally about 10 points below average, and that many had higher than average levels of anxiety and attention deficit problems. Relationship problems with a partner were the most frequent cause of stress for pregnant women, the research revealed.

One theory is that so-called "foetal programming" developed as an evolutionary strategy to prepare children to cope with life, on the basis that if the mother was highly stressed, the baby was likely to be born into a dangerous environment.